Packet switched network protocols, in which data is sent from one network node to another in the form of one or more discrete packets, have long been used in conventional wired networks to manage the exchange of data between network nodes. In wireless communications, it is sometimes advantageous to employ a protocol that allows for the transmission and receipt of data packets. One such environment is the wireless local area network (LAN) environment.
To facilitate the development and use of wireless LAN (“WLAN” or “WiLAN”) technology, the Institute of Electrical and Electronics Engineers (IEEE) has promulgated a series of standards, the IEEE Standard 802.11 series, to provide standardized specifications for the medium access control (MAC) and physical layer (PHY) for such a wireless LAN. IEEE Standard 802.11b provides the specifications for extending the PHY layer in the 2.45 GHz band.
IEEE Standard 802.11b prescribes a packet format in which each packet comprises a preamble portion, a header portion, and a data portion, transmitted in that order. In general, the preamble portion is used by the receiving system to recognize that a packet is being received and to perform synchronization operations to enable the receiving system to reliably receive and interpret the incoming packet. The header portion typically is used to provide to the receiving system information concerning the data portion that will follow, such as by indicating how much data is being sent in the packet and what signaling method is being used to transmit the data portion. The data portion, sometimes referred to as the “payload” of the packet, comprises all or a part of a message the sending system is communicating to the receiving system. The message may comprise, for example, a request for data available from the receiving system or data sent by the sending system in response to a request received from the receiving system. How the data is processed at the receiving system may be dictated, for example, by an application layer protocol or application program running on the receiving system.
In certain environments, such as in a wireless LAN environment operating under IEEE Standard 802.11b, more than one packet format may be used. For example, IEEE Standard 802.11b specifies a long preamble format and a short preamble format. The long preamble format may be needed, for example, for use with legacy or other systems that employ the 1 Megabit per second (Mbps) mode of operation under IEEE Standard 802.11b. The short preamble format, on the other hand, may be suitable for use with higher data rate modes of operation, such as for 2 Mbps, 5.5 Mbps, and 11 Mbps modes of operation.
Under IEEE Standard 802.11b, for both the long preamble format and the short preamble format the preamble comprises a first set of bits to be used for synchronization operations and a second set of bits, termed the “start frame delimiter” (SFD), which second set of bits the receiving system uses to recognize that the end of the preamble has been reached and the header portion of the packet is about to begin.
In certain implementations under IEEE Standard 802.11b, for example, or under other wireless communication protocols comprising more than one preamble format, it may be advantageous to determine dynamically, while the preamble is being processed (e.g., while synchronization operations are being performed), which preamble format is being used. For example, under IEEE Standard 802.11b, it may be advantageous to determine dynamically, during synchronization, that the long preamble format is being used. Under IEEE Standard 802.11b, the long preamble format comprises 128 bits for synchronization and a 16-bit SFD. By contrast, the short preamble format comprises just 56 bits for synchronization and a (different) 16-bit SFD. Therefore, based on dynamic detection of the preamble type the receiving system would know whether it could perform more extensive synchronization operations—because more bits would be available in the longer preamble format—in order to achieve better synchronization and channel sensitivity and thereby ensure more reliable interpretation of the received signal. This may be particularly important under a protocol such as IEEE Standard 802.11b, under which the long preamble format must be used with the lower data rate mode of operation (1 Mbps), which typically exhibits a lower signal-to-noise ratio (SNR) such that better synchronization may be required than for the higher data rate modes of operation, which typically have a higher SNR. Therefore, for situations in which a system may be required to be configured to receive and process data packets having more than one type of preamble, there is a need for a way to detect dynamically which preamble format is being used.